FEATURE ARTICLE – Effects of offshore renewables on the benthos

The latest in our series of expert features looks at the interrelationship between structures such as wind turbines and the seafloor and its organisms, as well as current scientific understanding on the subject.

Worldwide, offshore man-made structures are proliferating as a consequence of the growing interest in Blue Growth, including energy supply. Oceans are home to many types of such structures, for example oil and gas installations, pipelines and, more recently, a vast amount of renewable energy structures.

Particularly in light of extensive offshore wind farm development in shelf seas, offshore marine renewable energy developments (MREDs) are expected to affect the structure and functioning of marine ecosystems. More than 1500 turbines are already operational in the OSPAR region, with another 8800 either authorized or in application stages. The first wind farm in US waters was constructed just a few months ago. MREDs may therefore start affecting the marine ecology on larger spatial scales.

Current knowledge

MREDs represent the introduction of vast amounts of artificial hard substrate into a predominantly soft sediment environment. Wind turbines span the entire water column, thereby connecting surface water layers with seafloor sediments, which differentiate them from seafloor restricted natural (such as gravel beds) and artificial hard substrates (pipelines, ship wrecks). Moreover, several activities linked to the installation and operation of MREDs each have a specific ecosystem impact.

Large offshore structures have unique effects on the marine ecosystem. They induce changes in biodiversity, with repercussions on local as well as regional ecosystem functioning. MREDs provide habitat for a fouling community, often new to offshore regions. They serve as stepping stones for range-expanding (non-indigenous) species that may otherwise not be able to establish populations in a new area. The structures provide habitat and shelter for juvenile and adult pelagic and demersal species. They also alter biological processes, such as provision of new epifaunal prey items, and biogeochemical processes throughout the water column and on the seafloor. This can happen either directly through scouring effects and organic matter export from piles or indirectly through reduced stress from bottom fisheries.

The benthos plays a key role in the ecosystem, supporting numerous ecosystem goods and services such as long-term carbon storage and food resources for higher trophic groups such as fish, birds and mammals, including humans. An understanding of the ecological processes and patterns of the benthos (for example trophic interactions, nutrient cycling, and dispersion) is essential for managing ecological integrity key to the ecosystem and society. However, such understanding, particularly in relation to MREDs, is still in its infancy.

Understanding the cause-effect relationships behind the impacts of MREDs is essential for assessing the consequences for ecological functioning, and hence the relative meaningfulness of the impacts. Cause-effect relationships can be manifold and include abiotic and biotic changes; the latter ones can be direct or manifest in cascading effects along the food chain. The introduction of artificial hard substrate causes the settlement of fouling species, which become available as additional food sources for other species, potentially changing populations of fish and other higher trophic level species. Basic information on the local structural effects of MREDs, as available from the many ongoing monitoring projects, however generally fails in giving a profound insight into these cause-effect relationships.

Targeted field and laboratory studies should help fill these knowledge gaps for key ecological processes. Such studies can only be meaningful when performed at spatial scales that are ecologically relevant. The MRED effects onto cod population productivity for example can only be assessed when covering the full spatial extent of the cod stock's distribution, including its spawning, nursery, and feeding grounds.

Major value can be obtained from combining field and laboratory data and modelling exercises to get a detailed view on MRED cause-effect responses. This will further help to disentangle the effects through MREDs from the natural background variability and to get a comprehensive view on cumulative and in-combination impacts, and more importantly, how these impacts interact.

Further strengthened collaboration between industry and the scientific community is needed, as data sharing, in situ experiments, and data acquisition are beneficial to both. Ecological responses and processes can indeed be studied in greater detail, and an optimized use of the vast amount of data from both parties can potentially minimize future monitoring needs or at least make these more effective.

Understanding the MRED effects on ecological functioning and biodiversity will form a solid science base for ecologically-sound MRED development and the application of advanced management and conservation tools to enable the sustainable use of these ecosystems. Future research should provide ready-to-use information to support the further sustainable development of renewable energy by minimizing the ecological footprint and maintaining ecological integrity. Translating scientific findings into information of use to decision making-processes indeed remains key to WGMBRED's work. ​